The present invention relates to the technical field of cable installations, for example optical fibre or electrical, inside buried and/or submerged installation conduits. The term “cables” must also be taken to mean tubes or the like for conveying power, fluids and electrical, light or other signals. The term “cable” also concerns low, medium and high power electrical cables, single or three-phase.
Various methods are known for installing cables within conduits. These methods are defined below.
“Pulling” means that the cable is attached by its end to a sling already installed in the conduit. This sling is attached to the drum of a winch which winds the latter and enables the cable to be installed in the conduit, by sliding the cable within the said conduit.
“Pulling-pushing” means simultaneous drawing and pushing with the use of means for pushing the cable at its entry into the conduit, in order to reduce the traction forces on the cable head and to increase the installation distance. Means of driving the cable towards the half-way mark can also be provided.
“Blowing” means that the installation conduit is put under pressure, by means of a fluid, between the cable supply means and a fluidtight pig attached to the head of the cable. The pressure prevailing inside the conduit thus moves the pig, pulling on the cable head. The pressurisation is generally effected with air.
“Blowing-pushing” means the use of means for pushing the cable at its entry into the conduit, associated with the means used in the so-called “blowing” method. There also, an increase in the cable installation distance is obtained.
“Floating-pushing” and/or “carrying-pushing” means a technique in which a fluid, generally air or water, is injected into the conduit at a speed greater than the speed of movement of the cable, and it is the friction of the fluid on the external sheath of the cable which exerts the traction force. In this installation technique, the static pressure of the air or of the liquid inside the conduit has the effect of pushing the cable out of the conduit.
In certain “pulling-pushing” techniques, water is also injected into the conduit. The traction force comes from a pig fixed to the head end of the cable on which the pressure of water injected into the conduit is exerted. For this purpose, the pig is provided with one or more seals.
One of the advantages of the techniques of injecting water or other liquids is the reduction of the bearing force of the cable against the internal wall of the conduit due to the buoyancy and consequently the friction forces. The installation distance is however limited by the maximum permissible water pressure, which is determined by the maximum pressure which can be applied to the pig.
Because of the radii of curvature in the conduit, and the deformation in an oval shape of the profiles of the conduits, existing pigs are provided with a juxtaposed series of flexible rubber lips in a conical shape or in the form of disks. These flexible lips have the great disadvantage of not withstanding a very high pressure not normally exceeding a few bar, so that the pulling force of the pig is relatively limited.
In the light of these drawbacks, one aim of the invention is to provide a pig for installing cables in conduits which makes it possible to increase the cable installation distance, that is to say the length of the cable which can be inserted in a conduit.
It is advantageous to provide a cable installation pig able to be used in installation techniques in which a liquid, for example water, is injected into the cable conduit.
It is also advantageous to provide a pig for installing cables in a conduit which is reliable and economical and which reduces or even eliminates leakages of liquid.
It is advantageous to provide a cable installation pig which can also be used for extracting a cable from a conduit.
Aims of the invention are achieved by a pig for installing cables.